Assuming that this plant sale is something you don't want to put alot of cost into- I think Daves's idea of using window blinds would be the way I would go. Also if you are selling several of the same kinds of plants you could just hand out a printed flier for taking care of them. Your flier could contain basic info for iris, hosta, daylily or bulbs. That way you can just hand out one flier and cover most of the plants.
